1. Field of the Invention
The present invention relates to an electronic musical instrument which generates musical tones through digital signal processing and more specifically, to an electronic musical instrument capable of simulating the musical tones of natural musical instruments such as pianos, flutes, horns, strings, etc.
2. Description of the Prior Art
In recent years, the introduction of digital signal processing technique into electronic musical instruments has enabled the generation of high-quality musical tones. Provisional Japanese Patent Publication (Kokai) No. 52-107823 discloses an advanced electronic musical instrument, the manner of operation of which will be described with reference to a block diagram shown in FIG. 15. When a key on a keyboard 300 is depressed, an R-number memory 301 generates frequency information (hereinafter referred to as "an R-number") corresponding to the depressed key. A gate 302 is controlled by a clock pulse .phi.. An accumulator 303 adds the R-number repeatedly at every clock pulse .phi.. Therefore, the output S of the accumulator increases at every clock pulse .phi. from S=0, R, 2R, . . . , and after the output S has exceeded a constant N, a difference S-N remains in the accumulator 303. A waveform memory I 310 and a waveform memory II 320 provides waveforms of two systems addressed by the output S of the accumulator 303. Therefore, when the addressable area of the waveform memory I 310 and the waveform memory II 320 is N, the frequency f of the output is: EQU f=R/N.times.(frequency of clock pulse .phi.) (1)
Logarithmic waveforms log W.sub.1 and log W.sub.2 are stored beforehand in the waveform memory I 310 and the waveform memory II 320, respectively. A time function generator 330 generates a time function f(t). Indicated at 331 is a logarithmic converter (L/LG converter). An adder 311 adds: a.sub.1 +a.sub.2 and a subtractor 321 subtracts b.sub.1 -b.sub.2. Elements 312 and 322 are logarithmic/linear converters; (LG/L converters) element 341 is an adder, element 350 is an envelope generator, element 342 is a multiplier which multiplies the output of the adder 341 by the output of the envelope generator 350, element 343 is a D/A converter, element 344 is an amplifier and element 345 is a speaker. In operation, when the key is depressed, the time function generator 330 generates a time function f(t), and then the L/LG converter 331 converts the time function f(t) into log f(t). On the other hand, the waveform memory I 310 and the waveform memory II provide waveforms log W.sub.1 and log W.sub.2 of a frequency decided by Expression (1), respectively. Consequently, the adder 311, the subtractor 321, the LG/L converter 312 and the LG/L converter 322 provide log W.sub.1 +log f(t), log W.sub.2 -log f(t), W.sub.1 .times.f(t) and W.sub.2 /f(t), respectively. Therefore, the adder 341 provides W.sub.1 .times.f(t)+W.sub.2 /f(t), and then this output of the adder 341 is multiplied by the envelope signal generated by the envelope generator 350 in the multiplier 342. The result of the multiplication is converted into a corresponding analog signal by the D/A converter 343 and the analog signal is amplified by the amplifier 344 to drive the speaker 345 to generate a corresponding musical tone.
Since the above-mentioned constitution is calculated only to vary the mixing ratio of two waveforms with time, this constitution is unable to simulate the subtle variation in tone during the initial rising period (usually called the "attack period") of each sound. The portion of a sound waveform during the attack period will hereinafter be referred to as the "attack portion".